The Andean Court of Justice (ACJ), the judicial arm of the Andean Community, has emerged as one of the more active of a recent wave of newly created international or regional courts with jurisdiction over a regional trading and integration bloc. The ACJ is one of several such courts in South America, and the European Court of Justice (ECJ) can be understood as a model in terms of its jurisdiction and procedure. The ACJ has dealt with many of the same issues as the ECJ, especially what can be considered ‘constitutional’ issues. This paper is part of a larger study mapping legal reasoning in international courts, examining both the practice and the articulation of legal reasoning by these courts. First this paper examines the brief period of practice of the Central American Court of Justice (CACJ), the first modern international court, as to how the CACJ approached legal reasoning. A relatively small number of ACJ judgments are then selected and discussed in some detail on the basis of a qualitative criterion: their ‘constitutional’ and precedential importance in establishing fundamental principles operative in the legal system of the Andean Community. These principles relate to: supremacy of Andean Community law over the law of its Member States, the direct effect of Andean Community law in the courts of its Member States, and the external relations of the Andean Community. It is concluded that there is a superficial similarity of approach of the ACJ to the ECJ in their approach to legal reasoning, but there are substantial differences. The differences are that the ACJ has tended to be more explicit and does not argue for the specificity of Andean system to the same extent and it appears to accept the universalisability of interpretation. On the other hand, as with the ECJ, neither does it articulate the issue of levels of generality or the substantive reasons for different methods of interpretation. Influences across different regional integration blocs are likely to become a stronger feature of international judicial practice.